using System;

using System.Collections.Generic;

using System.Linq;

​

public class Student

{

    public string FirstName;

    public string LastName;

    public DateTime DOB;

    public int Age{

        get{

            return ((DateTime.Now - DOB).Days / 365);

        }

    }

    public Student(string firstName, string lastName, DateTime dob)

    {

        FirstName = firstName;

        LastName = lastName;

        DOB = dob;

    }

    public override string ToString(){

        return string.Format("{0} {1}", this.FirstName, this.LastName);

    }

}

​

public class Program

{

    public static void Main()

    {

        var students = GetSampleStudentList().OrderBy(x => x.DOB);

        foreach (var s in students)

        {

            Console.WriteLine(string.Format("Name: {0}, Age: {1}", s, s.Age));

        }

        Console.WriteLine("");

        Console.WriteLine(string.Format("Name: {0}, Exist: {1}", "john", FirstNameExists(students.ToList(), "john")));

        Console.WriteLine(string.Format("Name: {0}, Exist: {1}", "Ted", FirstNameExists(students.ToList(), "Ted")));

        Console.WriteLine("");

        var result = students.GroupBy(x => x.FirstName)

                .Select(x => new { 

                        FirstName = x.Key,

                        QtdStudents = x.Count()

                });

        foreach (var s in result)

        {

            Console.WriteLine(string.Format("Name: {0}, Qtd: {1}", s.FirstName, s.QtdStudents.ToString()));

        }

    }

    public static bool FirstNameExists(List<Student> students, string name)

    {

        return students.Where(x => x.FirstName.ToLower() == name.ToLower()).Count() > 0;

    }

​

    private static List<Student> GetSampleStudentList()

    {

        List<Student> students = new List<Student>();

        students.Add(new Student("John", "Smith", new DateTime(1978, 4, 16)));

        students.Add(new Student("Jack", "Smith", new DateTime(1978, 3, 01)));

        students.Add(new Student("John", "Rambo", new DateTime(1977, 3, 16)));

        students.Add(new Student("Jack", "Johnson", new DateTime(1981, 12, 13)));

        students.Add(new Student("John", "Jackson", new DateTime(1980, 1, 1)));

        students.Add(new Student("David", "Carson", new DateTime(1993, 5, 6)));

        students.Add(new Student("Steve", "Stephenson", new DateTime(1979, 7, 8)));

        return students;

    }

}